Electrical resistor and method of making same



March 31, 1959 L. H. BERKELHAMER 2,880,296

ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Filed Spt. 23. 1954 INVEN TOR.

fizrmflzr United States Patent ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Louis H. Berkelhamer, Chicago, Ill., assignor to David T. Siegel, Glencoe, 111.

Application September 23, 1954, Serial No. 457,945

4 Claims. (Cl. 201-64) The present invention relates to a novel electrical imrequiring them to withstand the effects of rapid tempera ture changes between wide limits and high humidity and requiring them to have a relatively long operating life and stability when exposed to severe temperature changes. It was suggested that the above difficulties might be overcome by replacing the ceramic core with a plastic core and enclosing the coils in a relatively thick body of plastic material identical to the plastic material of the core to minimize internal strains caused by temperature changes. However, this suggestion is not entirely satisfactory since such plastic cores or bobbins are relatively difiicult and expensive to produce. Plastic cores or bobbins have been made by machining which is'relatively clifficult as compared to similarly forming a ceramic bobbin and, moreover, such plastic bobbins cannot easily be made uniform in size and shape 'since even thermo setting plastics are deformed under the pressure of a cutting tool so that acurate machining thereof is very diflicult.

It is a primary object of the present invention to provide a novel electrical resistor and a novel method for making an electrical resistor so that the above mentioned difficulties are overcome.

More specifically, it is an object of the present invention to provide a novel electrical resistor having substantially improved operating characteristics and a novel method for making the resistor economically.

Other objects and advantages of the present invention will become apparent from the following description and the accompanying drawings wherein:

Fig. l is a perspective view illustrating a resistor embodying the principles'of this invention;-

Fig. 2 is an enlarged cross sectional view taken along line2-2inFi'g.I;

, a mineral filler such as talc.

' mixture for the body 26 will not be porous and pervious Fig. 10 is a perspective view of a modified form of the invention; and

Fig. 11 is a cross sectional view of the resistor shown.

in Fig. 10.

Referring now more specifically to the drawings wherein: like parts are designated by the same numerals throughout the various figures, an electrical resistor element 10 em-' for confining the coils after which machining operation.

the ceramic material is fired. In this manner a ceramic core or bobbin may be economically and accurately.

formed. Metal terminal members 18 and 20 are mounted on opposite ends of the ceramic core or bobbin and ends 22 and 24 of the wire are secured to these terminal mema bers as by spot welding.

In accordance with an important feature of the present invention the ceramic core and the wire coils are encased in a body 26 of plastic material which is thermally com-v patible with the ceramic core. the body 26 closes not only the outer periphery and the ends of the core but it also encloses the internal surfaces of the core, or in other words, the plastic body extends within the central aperture of the core.v With this structure of the wire of the resistor element is hermetically; sealed within the body 26 which is impervious to moisture and since the plastic material is thermally compatible with the ceramic material of the core or bobbin, .the re-. sistor element can withstand relatively rapid and severe temperature changes. It is believed'that this substantial improvement in the operating characteristics of the re-' sistor element results from a reduction in'intern'al strains which might cause the plastic body to check or'crack or which might otherwise injure the resistor element. It has been found that the substantially improved results mentioned are obtained when the body 26 is formed with an epoxy resin and a suitable hardening agent mixed with In order to assure that the to moisture it is important that the filler particles have afibrous or needle-like structure rather than a granular structure so that they may pack and interlock tightly together. It is believed that the interlocking of the relatively long needle-like or fibrous filler particles also helps to increase the resistance of the plastic body to cracking or checking. 1

, As set forth in the preceding paragraph the body- 26' I hermetically seals.v the wire of the resistorand as will Fig. 3 is a cross sectional 'view taken along line 13- 3 v Fig. 4 is a fragmentary cross sectional view taken along line 44 in Fig. 3; 3

Fig. 5 is a perspective view illustrating a partially completed resistor embodying the principles of this invention;

Fig. 6 is a side elevational view illustrating a partially completed resistor after it has been impregnated with a suitable resin;

Fig. 7 is a somewhat diagrammatic'view partially in cross section illustrating one step in the production of resistors embodying the principles of this invention;

Fig. 8 is a cross sectional view taken along line 8--8 in Fig. 7;

Fig. 9 is an elevational view illustrating the last step in the production of resistors embodying the principles of this invention;

be seen by referring to the drawings the-only possible. opening in this seal. iswhere prongs 28 and 30 of the terminal members project outwardly from the body."

The plastic material of the body 26 adheres to'the metal of the terminal members so that the seal is complete.v However, in accordance with the present invention any possibility of moisture entering the body 26 through the openings created by the prongs 28 and 30 is reduced to a minimum by reducing the cross sectional area of the prongs at the points where they pass out of the body 26. Thus, the prong 28 is provided with an opening or struck out section 32 which extends above and below the surface 34 of the body 26 and the prong 30 is provided with a similar opening 36. The plastic material enters into the. openings and between spaced sections 38 and 40 of the. prongs as shown best at 42 in Fig. 4. Thus, the opening through the body 26 for each prong is equal to the com bined areas of the prong sections 38 and 40 rather than to the entire cross sectional area of the prong. By providing the apertures in the prongs in the manner illustrated in the openings through the body 26 may be kept Patented Mar. 31, 1959 It should be noted thatv at. a. size. while. at. the same time the p on will have sufiicient strength and rigidity.

In manufacturing the resistor element the ceramic ore is. prefe ably formed ymac ning; he rod of green ceramic material after which the core is fired;- A. wire. having a y suitable insula ing. coatina ther o is then wound on the core or bobbin to provide one or more coils. and the Opposite end of the wire are, Secured to the termina members. which, oi course, are fastened to the. opposite ends of; the core. At this, stage of production the. resistor will appear substantially as illustrated. in Fig. 5... Subsequently and as an optional step, the coils may be impregnated, with a. suitable varnish or resin such as a silicone resin by utilizing known vacuum impregnation'methods. In the event the resistor element is to be impregnated. with. a silicone: resin or other suitable material the. prongs 2.8. and 30 of the terminal membersmay' first be dipped into and coated with solder. Then after the resistor element is; immersed in the impregnating material the impregnating material is cleaned from the prongs and the, prongs may again be dipped into solder. The resulting structure is illustrated best in Fig. 6 which shows solder coatings 44 and 46 on the ends of the. prongs while, substantially the re,- mainder ofthe. element, is PIovided with. aCQating of the silicone resin orother suitable impregnating; material. While other means of applying the solder may be employed, the foregoing "procedure insures that the solder coatings to. which wires maybe easily attached will. adhere to the prongs, since the initial, application of the solder is made while. the prongs are uncontaminated. by the silicone resin or other impregnating material.

In preparing the material for the body 26 a mineral filler such. as talc is first mixed with a suitable thermoplastic resin such. as an epoxy resin. If desired a coloring material may first be mixed with the filler. Then a hardener agent is added to the mixture and the mixture. is molded or cast around the core or bobbin and wire coils to provide. the body 26. In order to reduce any possibility of air bubbles being contained in the final plastic body 26. the mixture. is, preferably subjected to vacuum prior to or during the casting operation to eliminate air therefrom.

When casting the body 26, a plurality of the bobbins which have been processed to the point: illustrated. in either Figs. 5. or 6 may be arranged in a cavity mold 50 as shown in Figs. 7 and 8. A rod 52 is positioned so that it extends through the central apertures of the cores or bobbins and the diameter of the rod is substantially less than the diameter of the core apertures. so that the plastic material; may extend through the core apertures in the manner illustrated. After the bobbins have been properly positioned within the mold the mixture of; plastic resin and filler which has. been, prepared in the. manner set forth above is poured into the mold and again. subjected to a. vacuum to furtherreduce-possir bilityroffany. air bubbles appearing; in, the. finished product. The mold is then placed in an oven and; the plastic materialv is cured at an elevated temperature. Preferably the plastic material is cured by first heating it" for-a short. period of time, then chilling it and then reheating to finish curing. This procedure will impart to the plastic material proper resistance to cracking and checking. As will be understo'od there may be instances in which it is desirable to eliminate the chillingof the mold and plastic material after curing orat least when this; step is unnecessary and also in many cases the plastic material may be cured at room. temperature although this usually takes a considerably longer period of time. The final step in the process is illustrated in Fig. 9 wherein the plurality of bobbins have been removed from the mold and the plastic material has been severed to separate the individual electrical impedance or resistor elements.

In Figs. 10 and 11 there is illustrated a resistor unit 60 which embodies a modified form of this invention. The unit 60 includes a ceramic core 62, wire coils 64 and a plastic body 65. These elements are essentially identical and are. formed in substantially the same manner as the corresponding elements of the unit 10 and need not be described in detail. The unit 60 differs from the unit 10 in that terminal members 66. are provided instead of the above described terminal" members 18 and 20. Each terminal member 66. includes a metal cap 68 which is applied to the end of the core or bobbin and an axially extending wire or rod 70 butt welded to the cap. The opposite ends of the wire coils are, of course, connected to the caps. This construction simplifies the application of the. terminal members to the core or bobbin and the wires or rods; 70 require the smallest possible opening through the plastic body so that any danger of moisture entering the plastic body is reduced to a minimum- From the above description it is seen thatthe present invention provides a. novel electrical impedance or resistor element having a substantially increased working life and resistance to thermal shocks and moisture. In addition, it is seen that by utilizing the. novel, method of this invention the resistor elements may be. rapidly and relatively economically manufactured.

While the preferred embodiments; of the present invention have been illustrated and described herein it is obvious that many details may be changed without. departing from the spirit and scope of the appended claims.

The invention is claimed as follows:

1. An electrical impedance unit comprising a ceramic core, electrical impedance means on said core, a terminal member on said core connected to said impedance means, and a plastic body thermally compatible with said ceramic core encasing said ceramic core and-said impedance means and hermetically sealing said impedance means, said terminal member having a prong extending outwardly from said plastic body, and said prong having an opening therein extending inwardly andj outwardly from a surface of said plastic body.

2. A method of making an electrical pile resistor unit comprising the steps of providing a ceramic core having wire coil means would thereon, forming a mixture of epoxy resin and talc filler, said mixture being thermally compatible with said cera nic core, hermetically sealing said coil means by forming a bodyof said resin and filler around said ceramic core. and said coil means, and curing the, body formed by heating the bodypartially to cure the-v epoxy resin, then. chilling the body, and then reheating the body to complete the curing of. the epoxy resin.

3. An electrical, impedance unit. as. setforth in claim 1 wherein the plastic body comprises a resin with a mineral filler.

4. An electrical impedance unit as setiforth in claim 3 wherein the mineral filler comprises talc.

References Cited. in. the file. of. this. patent UNITED STATES PATENTS 1,938,674 Terwilliger Dec. 12, 1933 2,179,212 Ganci Nov. 7, 1939 2,491,688 Pickels Dec; 20, 1949 2,500,449 Bradley Mar. 14,, 1950 2,508,511 Goodman May 23, 1950 2,518,225 Dorst Aug, 8, 1950 2,685,016 Blackburn July- 27; 1954 2,745,170 Nirns May 15, 1956 

